1. Technical Field
A light-emitting device and the method for making the same are disclosed.
2. Description of the Related Art
Semiconductor light-emitting elements are wildly utilized in various applications, such as traffic lights, Blue-DVD of high density storage devices, green light RCLED (used in the internal communications and control system of the plastic optical fiber used in car) and medical devices (UV LEDs), etc. The increasing of light-emitting efficiency makes the application of the light-emitting elements spread extensively, such as optical display device (RGB edge-lit back light units) or rear-projection TV. Therefore, the main research topic is to increase efficiency of light-emitting element.
The light emitted from the light-emitting device is omnidirectional. In the application of optical modules, the utilization efficiency of light is limited because of the influence of the etendue. Therefore, one method to increase the efficiency of the light-emitting device is to make the light emitted from the light-emitting device directionally and reduce the divergence angle. E. Yablonovitch and S. John presented a dielectric material periodically arranged in ½ order of wavelength of the radio waves in 1987 that the wavelength is from far ultrared rays to visible light (300-700 nm). The behaviors of the radio waves in the highly arranged material are similar to electrons in crystals affected by the spacial structure, the period of the arrangement, the structure type, and the dielectric constant of the dielectric materials. By designation of the order of optical wavelength and the photonic bandgap of the dielectric material without the need of changing the chemical structure of the dielectric material, a new artificial crystal element of different optical properties is called photonic crystal (PC) is introduced. When it is applied to light-emitting diode (LEDs), the surface of the LED is etched to form a patterned structure of two-dimensional photonic crystal to restrain the light emitted randomly from the LED and increase the light emitting upwardly, so the divergence angle is decreased and the light efficiency is improved.
The material composition of the photonic crystals has periodic variations in the x-y space, and the planal observation of the structure in two-dimensional equivalent refractive index is shown in FIG. 1. The equivalent refractive index of the structure is represented two-dimensionally wherein n1 represent the refractive index after the interdiffusion of the quantum well and n2 represent the refractive index before the interdiffusion of the quantum well. The refractive index difference (n1-n2) is defined as Δn, Δn=nr+j*ni(1-1), wherein nr represents the real unit of the refractive index difference and ni represents the imaginary unit of the refractive index difference. The material composition of the active layer is periodically varied, and the parameters of nr and ni coexist where nr influences the light extraction efficiency and ni influences the internal efficiency of LED. A conventional method is to etch two-dimensional photonic crystals on the surface of the LED to form a two-dimensional equivalent refractive index plane wherein the Δn has nr only (ni is zero) so as to influence the light extraction efficiency and the divergence angle of the LED.
Another conventional method is using the laser holography apparatus and the semiconductor processes like photography, development and etching to form nano-sized islands on the ohmic contact layer of the LED to increase the light extraction efficiency.